| Autor: |
Degnan JR; U.S. Geological Survey, New England Water Science Center , 331 Commerce Way, Suite 2, Pembroke, New Hampshire 03275, United States ., Böhlke JK; U.S. Geological Survey , 12201 Sunrise Valley Drive, 431 National Center, Reston, Virginia 20192 United States., Pelham K; New Hampshire Dept. of Transportation, Bureau of Materials and Research , 5 Hazen Drive, Concord, New Hampshire 03301, United States., Langlais DM; Hoyle, Tanner & Associates, Inc., I-93 Exit 3 Project Field Office , 77 Indian Rock Road, Windham, New Hampshire 03087, United States., Walsh GJ; U.S. Geological Survey , Box 628, Montpelier, Vermont 05602, United States. |
| Abstrakt: |
Explosives used in construction have been implicated as sources of NO3(-) contamination in groundwater, but direct forensic evidence is limited. Identification of blasting-related NO3(-) can be complicated by other NO3(-) sources, including agriculture and wastewater disposal, and by hydrogeologic factors affecting NO3(-) transport and stability. Here we describe a study that used hydrogeology, chemistry, stable isotopes, and mass balance calculations to evaluate groundwater NO3(-) sources and transport in areas surrounding a highway construction site with documented blasting in New Hampshire. Results indicate various groundwater responses to contamination: (1) rapid breakthrough and flushing of synthetic NO3(-) (low δ(15)N, high δ(18)O) from dissolution of unexploded NH4NO3 blasting agents in oxic groundwater; (2) delayed and reduced breakthrough of synthetic NO3(-) subjected to partial denitrification (high δ(15)N, high δ(18)O); (3) relatively persistent concentrations of blasting-related biogenic NO3(-) derived from nitrification of NH4(+) (low δ(15)N, low δ(18)O); and (4) stable but spatially variable biogenic NO3(-) concentrations, consistent with recharge from septic systems (high δ(15)N, low δ(18)O), variably affected by denitrification. Source characteristics of denitrified samples were reconstructed from dissolved-gas data (Ar, N2) and isotopic fractionation trends associated with denitrification (Δδ(15)N/Δδ(18)O ≈ 1.31). Methods and data from this study are expected to be applicable in studies of other aquifers affected by explosives used in construction. |
